This invention relates generally to capacitive electromagnetic interference (EMI) filters. More particularly, the present invention relates to a novel electromagnetic interference (EMI) filter which is designed to attenuate one or more specific frequencies in order to provide electromagnetic compatibility of an electronic device (such as a cardiac pacemaker) while in the presence of an electromagnetic (EM) emitter operating at the same or similar frequencies (such as an electronic article surveillance (EAS) system).
Capacitive EMI low pass filters are well know in the art as described in the U.S. patents to Stevenson U.S. Pat. No. 4,424,551, Stevenson U.S. Pat. No. 5,333,095, Rodriguez U.S. Pat. No. 3,235,939, Coleman U.S. Pat. No. 4,247,881, and Duncan U.S. Pat. No. 4,152,540. Low pass filters may be single (single capacitive element) or multi-element (combination of capacitors with inductors or resistors), and are designed to allow low frequency signals to pass with little to no attenuation while at the same time providing a high degree of attenuation at higher frequencies.
An example of an effective low pass filter would be one operating on the 400 Hz power line of an aircraft. The filter would allow the 400 Hz power line frequency to pass through unimpeded while providing over 80 dB of attenuation to undesirable EM signals in the frequency range from 10 KHz to 10 GHz. Unfortunately, such a filter is physically quite large as it requires a number of inductive (L) and capacitive (C) elements. This is simply not practical for many electronic devices such as cardiac pacemakers, where space and weight are at a premium. Even if such a large filter could be fitted inside a cardiac pacemaker, the frequency range below 10 KHz is unprotected and is thereby potentially susceptible to EMI. Another key design constraint for implantable medical devices, which derive their energy needs from batteries, is energy conservation. Because of this, filter designs that employ resistors or low frequency dissipative elements are undesirable.
Broadband low pass EMI filters that are widely used in cardiac pacemakers are described in U.S. Pat. Nos. 4,424,551 and 5,333,095, the contents of which are incorporated herein. These types of single pole coaxial feedthrough filter capacitors are very small in size and very effective in attenuating EM signals in the frequency range from 8 MHz to 10 GHz. This frequency range includes cellular phones and many other sources of EMI. However, this type of filter when used alone is ineffective for attenuating EMI at very low frequencies (from DC to 8 MHz), such as the powerful EM field that is produced by certain electronic article surveillance (EAS) systems.
EAS systems are widely used throughout the world to deter theft in retail stores by detecting a tag or sensor placed on an article (for example a shirt or pair of shoes). Unless the tag is removed or degaussed, the sensing field will set off an alarm if the thief attempts to leave the retail store. EAS systems are produced by various manufacturers and typically operate from 73 Hz to 10 GHz. EAS systems operating at frequencies above 8 MHz in general do not interfere with cardiac pacemakers or implantable cardioverter defibrillators (ICDs). This is due to the effectiveness of the broadband type EMI filters described above in combination with substrate mounted chip capacitors inside of the medical device housing. However, most EAS systems in the United States operate at frequencies below 8 MHz because these tend to be the most effective at deterring theft (higher frequency EAS systems are easily defeated by a thief lining his or her handbag with aluminum foil or like shielding). The most widely used EAS system in the United States is manufactured by Sensormatic Inc. under the model name xe2x80x9cUltramaxxe2x80x9d, and employs a 58 KHz EM sensing field which is produced between two pedestals and has a strong magnetic component. There are also EAS systems operating at 68 KHz and 39.5 KHz.
The Sensormatic Ultramax 58 KHz EM field is burst modulated at a rate of approximately 60 Hertz. This is problematic in that human cardiac activity occurs in the frequency range from 10 Hz to 100 Hz. Accordingly, the sensing/monitoring circuits of pacemakers and ICDs are designed to detect cardiac signals in this frequency range. The 58 KHz EAS carrier signal can enter the pacemaker housing and input circuitry via the cardiac leads or pacemaker telemetry coils. Pacemaker and ICD circuitry contain non-linear circuit elements such as high-voltage protection diodes which can act as modulation detectors (like a single sideband detector). After the 58 KHz carrier encounters a non-linear circuit element, the detected 60 Hz modulation can then enter past the pacemaker band pass filters (which pass signals between 10 and 100 Hz) and be amplified. The pacemaker can then confuse the EAS modulation as cardiac electrical activity. In the worst case, the pacemaker may inhibit or skip beats because it confuses the EAS modulation for normal cardiac activity (a demand pacemaker inhibits or shuts off in the presence of a normal heart beat).
In a paper presented at the annual meeting of the North American Society of Pacing and Electrophysiology (NASPE), 1997, New Orleans, Dr. McGiver presented a paper which reported on numerous interactions between pacemakers and EAS devices. In addition, the FDA has also received a number of Medical Device Reports (MDRs) which report on interaction between pacemakers and EAS systems.
One approach to public safety would be to warn pacemaker wearers to walk quickly between or avoid loitering near EAS pedestals (some pacemaker patient manuals warn patients to exit quickly through the center of the pedestals). However, some of the latest architectural model EAS systems are designed to be placed out-of-sight under a floor or in cabinetry. Other counter top systems may be placed next to checkout lines. Warning pacemaker wearers to avoid retail stores is, of course, unrealistic and would create much patient anxiety. This is particularly true for the elderly (of whom many wear pacemakers) where shopping is an important recreational activity. Yet another approach would be to place warning signs in the front of stores with low frequency EAS systems. This too, however, would create anxiety and deter the elderly from an important facet of daily living.
Removal of EAS systems from retail stores is equally unacceptable. Retail theft is a major financial burden on society. EAS systems perform a vital role in keeping the costs of goods reasonable while allowing the public unimpeded access to retail stores.
Usually, management of EMI at low frequencies such as 58 KHz is accomplished by the twisting of lead wire pairs. In non-human implant electronic systems, twisting of lead wires is a common practice to cancel EMI currents that are induced due to EM fields. However, the lead systems used for human implant have not been designed with field cancellation as an aim. Designing new twisted lead systems for human implant is problematic due to technical challenges and the many years of redesign and subsequent reliability testing that would be required. For example, cardiac implant leads must withstand the rigors of millions of heart beat muscle contractions.
Accordingly, there is a need for an improved EMI filter designed specifically to protect pacemakers or ICDs from EAS systems. Such an EMI filter must be very small and lightweight to fit inside of an implantable medical device, and must not attenuate adjacent pacemakers/ICD telemetry signals or minute ventilation (MV) signals. These normal pacemaker operating signals are typically close in frequency to the EAS 58 KHz system with pacemaker telemetry examples including 75 KHz and 100 KHz. This close frequency spacing generally precludes the use of broadband filters. Further, such a novel EMI filter must have a form factor which readily will fit inside of the housing of a pacemaker or ICD, and must be highly reliable but reasonable in cost. Additionally, such an EMI filter must be placed very close to the point of entry of the EAS signal into the pacemaker/ICD circuitry. It is very important to bypass or reject the EAS carrier before it is detected. Once the EAS signal encounters a non-linear circuit element, its 60 Hz modulation will be detected. Once detected, it is simply not practical to filter the modulation as this is a frequency which the implantable medical device monitors as normal cardiac activity. Finally, such a filter is needed which is capable of both common and differential mode rejection. This need varies with the design of the implantable medical device and placement/design of the implant lead wires. The present invention fulfills these needs and provides other related advantages.
The present invention resides in a novel electromagnetic interference (EMI) filter that is designed to attenuate one or more specific frequencies in order to provide electromagnetic capability of an electronic device while in the presence of an electromagnetic emitter operating at the same or similar frequencies. The EMI filter comprises, generally, a broadband electromagnetic interference filter associated with one or more leads of an electronic device and capable of attenuating a range of frequencies, and an inductor-capacitor (L-C) series resonant notch electromagnetic interference filter associated with the leads of the electronic device and capable of attenuating a specific frequency outside the attenuation range of the broadband EMI filter. This combined notch and low pass filter arrangement is particularly effective in an implantable medical device such as a cardiac pacemaker or an implantable cardioverter defibrillator (ICD) against passage of external interference signals, such as those caused by digital cellular phones and electronic article surveillance systems operating at low frequencies.
More specifically, the broadband EMI filter typically comprises a capacitive low pass filter such as a feedthrough filter capacitor which forms at least a portion of a hermetic terminal for an implantable medical electronic device. In various embodiments of the invention, the notch EMI filter comprises an L-C series resonant circuit disposed between each lead of the electronic device and a ground and/or between two leads of the electronic device. Further, the notch EMI filter may comprise paired L-C series resonant circuits such that the respective value of the inductor and the capacitor elements are reversed in order to provide both common mode and differential mode attenuation. Moreover, the notch EMI filter may comprise a plurality of notch electromagnetic interference filters associated with the leads of the electronic device and which are capable of attenuating a plurality of specific frequencies outside the attenuation range of the broadband EMI filter.
In another preferred form of the invention, a capacitive element of the notch EMI filter is integrated with the feedthrough filter capacitor. In this case the EMI filter includes a casing of dielectric material through which the leads of the electronic device extend. A first set of electrode plates is disposed within the casing in conductive relation with the leads of the electronic device. A second set of electrode plates is also disposed within the casing in non-conductive relation with the leads of the electronic device and in an alternating stack with the first set of electrode plates. A first conductive termination surface is conductively coupled to the second set of electrode plates. The first and second sets of electrode plates form the broadband EMI filter. Further, a third set of electrode plates is disposed within the casing in conductive relation with the leads of the electronic device. A fourth set of electrode plates is disposed within the casing in non-conductive relation with the leads of the electronic device and in an alternating stack with the third set of electrode plates. A second conductive termination surface is conductively coupled to the fourth set of electrode plates, wherein the third and fourth sets of electrode plates form the capacitive element of the notch EMI filter. Finally, an inductor is conductively coupled to the second conductive termination surface to form an inductive element of the notch EMI filter.
The inductor may be conductively coupled to the first conductive termination surface or between the second conductive termination surfaces of two leads of the electronic device. The inductor may comprise a chip inductor or a wound wire solenoid on a ferrite core of toroidal wound wire construction. The second conductive termination surface may be disposed in through-holes within the dielectric casing, or may comprise a metalized pad on an exterior surface of the dielectric casing.
The present invention further resides in a process for providing electromagnetic capability of an electronic device while in the presence of an electromagnetic emitter operating at the same or a similar frequency or frequencies. In a preferred form of the invention, the process provides electromagnetic compatibility of an implantable electronic medical device while in the presence of an electronic article surveillance (EAS) device operating at the same or a similar frequency or frequencies. In this case, the process steps include (1) associating a low pass broadband electromagnetic interference (EMI) filter with one or more leads of the implantable electronic medical device, and (2) attenuating one or more specific frequencies passing through the feedthrough filter capacitor and genera ted by the EAS device utilizing a notch electromagnetic interference (EMI) filter. Preferably, the broadband EMI filter comprises a feedthrough filter capacitor that forms at least a portion of a hermetic terminal for the implantable medical electronic device.
The novel EMI filter of the present invention is, thus, very small and lightweight so as to fit in side the implantable medical device, highly reliable, capable of both common and differential mode reject ion, an d may be placed very close to the point of entry of the EAS signal into the implantable medical device circuitry.
Other features and advantages of t he present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.